Method for remedial action in the event of the failure of the compressor bypass valve in a fuel cell system

ABSTRACT

A system and method for controlling the speed of a compressor that provides air to the cathode side of a fuel cell stack in the event that a cathode by-pass valve fails. If a by-pass valve failure is detected, a failure algorithm first disengages the normal flow and pressure algorithms used to control the airflow to the cathode side of the stack. Next, the failure algorithm opens the cathode exhaust gas valve to its fully opened position. Then, in response to a stack power request, the compressor control will be put in an open-loop control where a look-up table is used to provide a particular compressor speed for a power request. An airflow meter will measure the airflow to the stack, and the stack current will be limited based on that airflow.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for controllingthe cathode airflow to a fuel cell stack in the event of a failure ofthe cathode input by-pass valve and, more particularly, to a system andmethod for controlling the airflow to the cathode side of a fuel cellstack in the event of a cathode input by-pass valve failure thatincludes maintaining a cathode exhaust gas valve in an open position,setting the speed of the stack compressor to open-loop control using alook-up table, measuring the airflow to the stack and limiting thecurrent output from the stack based on the measured airflow.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electrochemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free hydrogen protons and electrons. The hydrogenprotons pass through the electrolyte to the cathode. The hydrogenprotons react with the oxygen and the electrons in the cathode togenerate water. The electrons from the anode cannot pass through theelectrolyte, and thus are directed through a load to perform work beforebeing sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA). MEAs are relatively expensive to manufactureand require certain conditions for effective operation.

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For example, a typical fuel cell stack for avehicle may have two hundred or more stacked fuel cells. The fuel cellstack receives a cathode input gas, typically a flow of air forcedthrough the stack by a compressor. Not all of the oxygen is consumed bythe stack and some of the air is output as a cathode exhaust gas thatmay include water as a stack by-product. The fuel cell stack alsoreceives an anode hydrogen input gas that flows into the anode side ofthe stack.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack.

The stack air compressor cannot operate at all flow and pressurecombinations required by a fuel cell stack. Therefore, a fuel cellsystem will typically include a cathode by-pass valve that allows atleast some of the airflow to by-pass the fuel cell stack and flowdirectly to the cathode exhaust. For example, there are points in theoperating range of a fuel cell stack that require less cathode airflowthan the compressor is capable of delivering at its minimum speed.During these conditions, the by-pass valve is used to redirect some ofthe compressor airflow to the exhaust.

In some fuel cell systems, the cathode by-pass valve defaults to theopen position so that if the valve were to fail, much of the compressorairflow would be sent to the system exhaust. In response to the by-passvalve failure in the open position, the compressor will increase itsspeed until its maximum speed is reached in an attempt to deliver theairflow necessary to meet the stack power request. Further, with theby-pass valve in the open position, the system algorithms would attemptto increase the pressure in the fuel cell stack by closing the cathodeexhaust gas valve. With the cathode exhaust gas valve closed to attemptto control the stack pressure, very little, if any, airflow will get tothe stack because it will be flowing out the cathode exhaust through theby-pass valve.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system andmethod are disclosed for controlling the speed of a compressor thatprovides air to the cathode side of a fuel cell stack in the event thata cathode by-pass valve fails. If a by-pass valve failure is detected, afailure algorithm first disengages the normal flow and pressurealgorithms used to control the airflow to the cathode side of the stack.Next, the failure algorithm opens the cathode exhaust gas valve to itsfully opened position. Then, in response to a stack power request, thecompressor control will be put into an open-loop control where a look-uptable is used to provide a particular compressor speed for a powerrequest. An airflow meter will measure the airflow to the stack, and thestack current will be limited based on that airflow.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fuel cell system employing aprocess for controlling a cathode input airflow from a compressor in theevent of a failure of a cathode by-pass valve, according to anembodiment of the present invention; and

FIG. 2 is a flow chart diagram showing a process for controlling thecompressor in response to a cathode by-pass valve failure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa system and method for controlling a compressor in response to afailure of a cathode by-pass valve in a fuel cell system is merelyexemplary in nature, and is in no way intended to limit the invention orits applications or uses.

FIG. 1 is a schematic block diagram of a fuel cell system 10 including afuel cell stack 12. The fuel cell system 10 also includes a compressor14 driven by a motor 16 that provides an airflow to a cathode side ofthe fuel cell stack 12 on a cathode input line 18. A temperature sensor20 measures the temperature of the cathode input air into the stack 12and an RH sensor 22 measures the relative humidity of the cathode inputair into the stack 12. The humidity of the cathode input air is providedby a water vapor transfer (WVT) unit 24 that receives the cathodeexhaust gas on cathode exhaust line 26 that typically has a humidity of100% or more. Water and water vapor from the cathode exhaust gas is usedin the WVT unit 24 to humidify the cathode input air that is provided onthe input line 18. A pressure sensor 48 in the cathode exhaust line 26measures the pressure of the cathode side of the stack 12. The cathodeexhaust gas is output from the WVT unit 24 on system output line 28 andis controlled by a cathode exhaust gas valve 30. For those times when itmay not be desirable to humidify the cathode input air, the cathodeinput air can be directed around the WVT unit 24 on by-pass line 32through a by-pass valve 34.

The cathode input air from the compressor 14 is sent to a heat exchanger36 that reduces the temperature of the air that has been heated as aresult of it being compressed by the compressor 14. Additionally, theheat exchanger 36 can provide heat to the cathode input air duringcertain times, such as cold start up, to heat the fuel cell stack 12more quickly. A pressure sensor 50 at the output of the compressor 14measures the discharge pressure of the compressor 14. The cathode inputair from the heat exchanger 36 is sent through a flow meter 38, such asa mass flow meter, that measures the flow of the cathode input air tothe stack 12. As is well understood to those skilled in the art, theflow of the cathode input air to the stack 12 needs to be tightlycontrolled to provide the proper cathode stoichiometry so that too muchair is not provided to the stack 12 which would have an adverse dryingeffect on the membranes within the fuel cells in the stack, or toolittle air that can cause fuel cell instability as a result of oxygenstarvation. A temperature sensor 40 measures the temperature of thecathode input airflow to control the heat exchanger 36 and a valve 42controls the amount of cathode air that flows into the WVT unit 24 orby-passes the WVT unit 24 on the by-pass line 32.

The flow of air from the compressor 14 to the cathode side of the stack12 is controlled based on the stack current demand and the stackpressure. Being able to control the speed of the compressor 14 toprovide the exact amount of air for the desired cathode stoichiometry istypically not possible. Therefore, a cathode by-pass valve 44 isprovided that proportionally controls the amount of cathode input airthat by-passes the stack 12 or flows to the stack 12 through the heatexchanger 26. The cathode air that by-passes the stack 12 flows throughby-pass line 46 and directly to the cathode exhaust gas line 28.

The present invention proposes a process for controlling the speed ofthe compressor 14 in response to a failure of the by-pass valve 44. Asdiscussed above, typically the by-pass valve 44 will default to acompletely open position if it fails. Further, in order to control thepressure of the stack 12, normal control system algorithms would closethe exhaust valve 30. However, with the exhaust valve 30 in the closedposition, little or no air will flow to the stack 12 because of the lowflow resistance path through the open by-pass valve 44. Therefore, thepresent invention disengages the normal flow and pressure algorithms ofthe system 10, and opens the exhaust gas valve 30 so that at least someairflow from the compressor 14 will go through the stack 12. Further,the control of the compressor 14 will be switched to an open-loopcontrol where a power request from the stack 12 will cause thecompressor 14 to operate at a certain predetermined speed from, forexample, a look-up table. At this speed, the airflow through the stack12 is measured by the flow meter 38, and the maximum current draw fromstack 12 is set based on that airflow. Once the airflow to the stack 12is known, then other system processes can be controlled accordingly,such as the amount of hydrogen provided to the anode side of the stack12. Because the amount of air provided to the stack 12 will typically below with the by-pass valve 44 open, the vehicle will typically be in a“limp home” where the amount of power able to be provided by the stack12 will be minimal. The fuel cell system 10 includes a control systemsuch as a controller 52 that controls the fuel cell system 10 includingthe speed of the compressor 14 and the position of the valves 44 and 30,as discussed in more detail below.

FIG. 2 is a flow chart diagram 60 showing the operation for controllingthe speed of the compressor 14, such as by using the controller 52 asdiscussed above. At box 62, the flow control algorithms will detect afailure of the by-pass valve 44, and will disengage the primary airflowand pressure control algorithms at box 64. The algorithm will then setthe cathode exhaust valve 30 to its fully open position at box 66. Thealgorithm will then determine a stack power demand at box 66, and thespeed of the compressor 14 will be controlled at box 70 using anopen-loop control and a look-up table for the demand. Particularly, fora particular stack power request, the speed of the compressor 14 will beset at some predetermined speed for that request. The airflow to thestack 12 is then measured by the flow meter 38 at box 72, and the stackcurrent is limited at box 74 based on the measured airflow.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A method for controlling the speed of acompressor that provides an airflow to a cathode side of a fuel cellstack, said method comprising: determining that a cathode by-pass valvehas failed in a completely open position that controls the flow of airfrom the compressor to a cathode exhaust without going through thestack; disengaging primary airflow and pressure control algorithms inresponse to the valve failure; setting a cathode exhaust valve to anopen position; determining a stack power request from the fuel cellstack; measuring an airflow from the compressor to the fuel cell stack;and controlling the compressor using an open-loop control where thespeed of the compressor is set based on the stack power request and themeasured airflow to the stack is used to determine a maximum currentdraw from the stack.
 2. The method according to claim 1 wherein using anopen-loop control includes determining the compressor speed from alook-up table.
 3. The method according to claim 1 wherein measuring theairflow includes using a mass flow meter.
 4. A method for controllingthe speed of a compressor that provides an airflow to a cathode side ofa fuel cell stack, said method comprising: determining that a cathodeby-pass valve has failed that controls the flow of air from thecompressor to a cathode exhaust without going through the stack;disengaging primary airflow and pressure control algorithms in responseto the valve failure; determining a stack power request from the fuelcell stack; and controlling the compressor using an open-loop controlwhere the speed of the compressor is set based on the stack powerrequest.
 5. The method according to claim 4 wherein determining that acathode by-pass valve has failed includes determining that a cathodeby-pass valve has failed in a completely open position.
 6. The methodaccording to claim 4 further comprising setting a cathode exhaust valveto an open position when the control algorithms are disengaged.
 7. Themethod according to claim 4 further comprising measuring an airflow fromthe compressor to the fuel cell stack where the measured airflow to thestack is used to determine a maximum current draw from the stack.
 8. Themethod according to claim 7 wherein measuring the airflow includes usinga mass flow meter.
 9. The method according to claim 4 wherein using anopen-loop control includes determining the compressor speed from alook-up table.